Urbanzization represents one of the most widespread human disturbances to the landscape in the world. Urban environments are often associated with changes in biotic and abiotic factors (e.g. temperature, pollution, pollination), which have driven adaptive evolutionary responses across a diverse range of taxa. In addition, fragmentation associated with urbanization has altered patterns of gene flow among urban populations and increased the strength of genetic drift in smaller, more isolated urban poulations. As a result, urban populations often show evidence of reduced genetic diversity and are frequently differentiated from nearby rural populations. Despite advances in our understanding of the effects of urbanization on selection, gene flow and genetic drift, most studies only focus on the effects of urbanization on single evolutionary mechanisms. Thus, the relative importance of selection, gene flow and genetic drift in influencing the structure of urban populations in any one system remain largely unknown.
Our lab has previously documented widespread clines in the frequency of hydrogen cyanide (HCN) along urban-rural gradients across multiple cities, with lower frequencing of HCN in urban populations. While previous work has identified urban-rural gradients in snow depth and minimum winter temperature as an important selective agent producing these clines, the importance of gene flow and genetic drift in either constraining or facilitating the formation of phenotypic clines in cyanogenesis remain unexplored. In an upcoming paper, Johnson et al. (2018, Proc B, in press) use microsatellite genotyping of multiple individuals along urban-rural transects across 8 cities to examine changes in neutral genetic diversity and patterns of genetic differentiation associated with urbanization. In this project, I use a subset of this data from two cities (Acton and Fergus, Ontario) to perform some basic population genetic analysis that would form the first step in assessing the importance of gene flow and genetic drift in influencing allele frequencies along urban-rural gradients. One of these cities (Fergus) shows a clines in the frequency of HCN whereas the other (Acton) does not. Specifically, I address the following questions:
The data used in this project is a subset of the data presented in an upcomming paper in the Proceeding of the Royal Society B by Johnson, M. T. J. et al. (2018, in press). In the paper, they genotype 16 microsatellite loci from each of 10 individual T. repens plants from 14 populations spanning an urban to rural transect across 8 cities. Thus, they genotyped a total of 1,120 individuals. In this project, I use only a subset of this data and include the microsatellite data from 280 individuals representing data from 2 cities, namely, Fergus and Acton, Ontario (28 populations total, see Fig. 1 for example transect).